Oil, natural gas, etc. are produced in the North Sea, Alaska, and other arctic areas in many cases. For this reason, oil country tubular goods and line pipe which are used for conveyance of the produced oil, natural gas, etc. are required to have low temperature toughness.
In the past, as oil country tubular goods and line pipe, seamless steel pipe and electric resistance welded steel pipe have been used. In recent years, from the viewpoint of cutting drilling costs, demand for electric resistance welded steel pipe has been rising. Along with this, various art is being developed for electric resistance welded steel pipe excellent in low temperature toughness. Specifically, improvement of toughness of the material by limiting the finishing temperature of the hot rolling process and the coiling temperature, refinement of the crystal grains by the addition of Nb and V, heat treatment of the pipe bodies after pipemaking, etc. may be mentioned.
In recent years, along with the increasing harshness of the environments of use of electric resistance welded steel pipe, demand for SSC resistance and low temperature toughness has been rising. To satisfy these demands, the method of adding Ca is known to be effective.
PLT 1 discloses the art of making the S concentration in the molten steel 7 ppm or less, then treating this molten steel with Ca to control the ratio of the Ca concentration and oxygen concentration to 1.19<([Ca]/[O])<2.11 and controlling the speed of addition of Ca to 0.023<V[kg/(t·min)]<0.7.
Research in recent years has revealed that electric resistance welded steel pipe to which Ca has been added sometimes remarkably falls in toughness of the electric resistance weld zone compared with the base metal part. The cause of this drop in toughness of the electric resistance weld zone has been found to be the inclusions present at the electric resistance weld zone and its vicinity being pressed from both sides and deforming to plate shapes. Further, the inclusions which deform to the plate shapes were analyzed for chemical composition. As a result, they were found to be composite inclusions which have molecular ratios of composition of mCaO.nAl2O3 (where, m, n are integers, same below).
PLT 2 discloses as art which solves the problem of the drop in toughness of the electric resistance weld zone the art of making the total amount of Si, Mn, Al, Ca, and Cr which are contained in inclusions with a circle equivalent diameter of 20 μm or more in the inclusions which are present at the electric resistance weld zone of electric resistance welded steel pipe a mass % of 20 ppm or less with respect to the total amount of the electric resistance weld zone which contains the base iron.
PLT 3 discloses the art of making the total amount of Si, Mn, Al, Ca, and Cr which are contained in inclusions with a circle equivalent diameter of 2 μm or more in the inclusions which are present at the electric resistance weld zone of electric resistance welded steel pipe a mass % of 99 ppm or less with respect to the total amount of the electric resistance weld zone which contains the base iron.